Integrated circuits that include devices ensuring reliable write access to non-volatile memories are known from the prior art. In particular, the use of a reprogrammable non volatile memory fitted with a circuit checking the programming voltage level which powers the memory in order to prevent corrupted data being written in the memory if the programming voltage is insufficient, is known from U.S. Pat. No. 5,943,263. This type of solution, based on checking the supply voltage is not suitable for the RFID domain, i.e. transmissions of data and energy by radio frequency. Indeed, in the case of a transponder circuit, it is not the voltage that powers the non-volatile memory which is indicative of the possibility of performing a reliable write operation, but rather the energy received from by the receiving antenna of the transponder circuit.
There is known from U.S. Pat. No. 5,925,139, a circuit for checking the high voltage obtained by the voltage booster circuit, such high voltage being necessary for performing a reliable write operation in a non volatile memory. This document discloses here a circuit checking the voltage necessary inside the memory and not the available energy received by an antenna for ensuring sufficient power supply to the memory in which one wishes to write. This document has the drawback of having to switch on the booster circuit in order to perform a check prior to the write operation. This check can be falsified due to the previously stored energy, which is undesirable in the case of a transponder circuit where the available energy is directly limited by the energy received.
There is also known from U.S. Pat. No. 6,166,960 a device for preventing corrupted data being written in a non-volatile memory in the event that the supply voltage is insufficient. In order to do this, the voltage booster is switched on usually by being connected to a charge representing the memory plane, and if the high voltage generated by the voltage booster is insufficient, then the write operation into the memory will not be authorised. For the same reasons as stated above, the assembly described in this document is not suitable for a transponder circuit type application.
The various existing solutions proposed in the prior art rely on two principles, one consisting in measuring the value of the supply voltage and the other consisting in switching on the voltage booster necessary for the memory to operate, in particular for write operations in the memory, and in measuring the voltage supplied to the output of the voltage booster.
However, neither of these two solutions is applicable to a transponder circuit since, in one case, measuring the supply voltage is not indicative of the energy available in the transponder circuit, and, in the other case, the evaluation period has to last as long as a real cycle for a write operation and takes account of any already stored energy.
This is why a completely different approach has to be developed to resolve the problem. Data has to be stored in a non-volatile memory without being corrupted. Any deletion or write operation in a memory requires a certain amount of energy. In the case of a transponder circuit, because of a limited energy storage capacity, or no available storage capacity, the energy essentially has to come from the radio frequency link which has a random value. Indeed, the periods of communication between the transponder circuit and the base station are interspersed with periods of rest during which the electromagnetic field is absent. Naturally, since the communication distance may vary between the transponder circuit and the base station, the energy received by the transponder circuit is thus also variable. Moreover, the amount of energy transmitted by the base station and received by the transponder circuit also depends upon the mutual position of the transmission and reception antennae. It will be noted that the small energy storage capacity that may be present in the transponder circuit is only provided for absorbing the peaks in consumption that appear at certain particular moments during a memory write operation.
It is also known that a write operation that has started cannot be interrupted before the end without endangering the integrity of the data to be stored. In the case of a transponder circuit, the potential risk of interrupting a write operation in progress is the lack of energy available to bring the operation to a normal end. It is thus imperative to know in advance, i.e. prior to starting a write operation, whether there will be enough energy to complete the operation.
It will also be noted that in order to resolve the problems linked to application to a transponder circuit, certain constraints also limit the possible applicable solutions, including:
First of all, it is not possible to accumulate reserve energy to ensure total completion of a write operation. The transponder circuit may be fitted with a battery, i.e. in the case of an “active” transponder, but the latter may be run down or simply absent, i.e. in the case of a “passive” transponder, and in any event it is not economical from the point of view of the compactness and price of components to add an additional capacitor to the final assembly in order to store the required energy. Indeed, the use of a minimum number of components outside the circuit is one of the objectives constantly sought for this type of application.
Secondly, the voltage level across the rectified and filtered supply voltage from the electromagnetic field received by the antenna is not indicative of the energy available for performing a complete write operation. The charge is stored in a filtering capacitor whose value is too low to store enough energy. Moreover, the voltage from the antenna is limited by an over voltage protector device which delivers a practically identical voltage over a large range of received energy whether or not it is sufficient for a memory write operation. Thus, even for a level of received energy that is too low, the antenna protector device or limiter enters into action and supplies a clipped voltage at a practically constant value.